Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Optical metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including ellipsometry, scatterometry, and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty.
Optical ellipsometry has long been recognized as an effective, non-destructive measurement technique that provides accurate characterizations of semiconductor and other materials, surface conditions, layer composition and thickness, and overlying oxide layers. In particular, ellipsometry has proven useful to evaluate thickness, crystallinity, composition, and index of refraction characteristics of thin films deposited on semiconductor or metal substrates.
An ellipsometer probes a sample with a light beam having a known polarization state. The light beam is reflected at non-normal incidence from the surface of the sample. Upon reflection, the polarization state of the beam is modified depending upon the properties of the sample. By accurately measuring the polarization state of the reflected beam and comparing it to the original polarization state, various properties of the sample can be ascertained.
In spectroscopic ellipsometry, either the probing wavelength is changed and the ellipsometric measurement is repeated at each new wavelength, or the probe beam contains a multiplicity of wavelengths and the reflected beam is detected with spectral resolution. Spectroscopic ellipsometry is advantageous for characterization of multi-material samples formed in stacked layers. The different depth penetrations and spectral responses that depend on the material and wavelength of light provide additional information about a sample that is not available from single wavelength ellipsometers.
Many configurations have been proposed to measure the change in polarization state that occurs upon reflection. In one type of ellipsometer only two optical elements are used, a polarizer and an analyzer, one of which is held fixed and the other rotated. Such an ellipsometer, commonly called a rotating-polarizer or rotating-analyzer ellipsometer, is termed “an incomplete” polarimeter, because it is insensitive to the handedness of the circularly polarized component and exhibits poor performance when the light being analyzed is either nearly completely linearly polarized or possesses a depolarized component.
Limitations of rotating-polarizer and rotating-analyzer ellipsometers are reduced by including a rotating compensator placed between the polarizer and the analyzer. The compensator can be placed either between the sample and the polarizer, or between the sample and the analyzer. The compensator is an optical component that delays the light polarized parallel to its slow axis relative to light polarized parallel to its fast axis. The delay is proportional to the refractive index difference along the two directions and the thickness of the plate.
Compensators are most easily implemented in highly collimated beams of light. The highly collimated beam transmitted through the compensator acquires a uniform delay across its wavefront. This uniformity is generally desired for simplicity of analysis. Various compensator designs exist for use with highly collimated beams. By way of example, a compound zeroth order waveplate is used in the OP2xxx-OP9000 model family of Beam Profile Ellipsometers (BPE) manufactured by KLA-Tencor Corporation, Milpitas, Calif. (USA). The waveplate is an air-spaced, quartz bi-plate that is anti-reflection coated. A compound zeroth order waveplate is also used in the OP5xxx-OP7xxx model family of Spectroscopic Ellipsometers (SE) manufactured by KLA-Tencor Corporation, Milpitas, Calif. (USA). This waveplate is an air-spaced, magnesium fluoride (MgF2) bi-plate. In another example, a MgF2monoplate is employed in the OP9000 family of spectroscopic ellipsometers manufactured by KLA-Tencor Corporation, Milpitas, Calif. (USA). All of these examples employ relatively thick (on the order of one millimeter) waveplates that are suitable for use with highly collimated light, but are not generally suitable for uncollimated light.
Other compensator designs include a Berek compensator, Fresnel rhomb, K-prism, and Soleil-Babinet compensator. All of these designs are sensitive to field angle and are only suitable for use within an ellipsometer employing highly collimated light.
In some examples, compensators are used in non-collimated beams of light. Different incident angles on the compensator have different directions of propagation inside the compensator, and thus generate different phase shifts and amplitudes. This results in a transmitted beam with a range of polarization states across the wavefront. This is described in greater detail in “Exact Theory of Retardation Plates by D. A. Holmes, J. Opt. Soc. Am, 54 (1964) 1115-1120, the entire content of which is incorporated herein by reference. To reduce the effect of varying incident angles, the thickness of the compensator elements may be reduced. For example, a thin (approximately ten micrometers), uniaxial quartz monoplate compensator is employed as part of a single wavelength elliposometer (SWE) incorporated into the Aleris product family manufactured by KLA-Tencor Corporation, Milpitas, Calif. (USA).
In general, thin, zeroth order waveplates are able to accommodate a larger field of view. But, these waveplates are typically thinner than the coherence length of the incident light. As a result, they suffer from phase and transmittance oscillations as a function of wavelength. This is a particular problem for ellipsometers operating with broadband light. Anti-reflection coatings reduce the amplitude of these oscillations, but typically only work well in a short wavelength range. Furthermore, broadband anti-reflection coatings can stress the waveplate, decreasing the retardation uniformity across the clear aperture. In general, free-standing, ultra-thin zeroth order waveplates cannot withstand the stress of even simple coatings. In the aforementioned monoplate compensator examples, the waveplate crystals (i.e., quartz and mica, respectively) are bonded to a thick (on the order of one millimeter) substrate of borosilicate (BK7) glass to provide mechanical stability and support for the crystals coated with anti-reflection coatings. Current manufacturing processes enable successful bonding of a very thin (approximately 10 micrometer) quartz crystal monoplate to the BK7 substrate. But, BK7 does not transmit ultraviolet light, and thus is unsuitable for use within a broadband ellipsometer including ultraviolet light.
Other compensator designs have been proposed for use with non-collimated light, including designs by Pancharatnam, Becker, Lyot, and designs incorporating biaxial polymers. These designs are described in greater detail in S. Pancharatnam, Proceedings of the Indian Academy of Sciences, A41, 130 (1955) and P. Yeh, C. Gu, “Optics of Liquid Crystal Displays,” Chapters 4.7-4.10, Wiley (2010). The contents of each are incorporated herein by reference in their entireties.
The designs of Pancharatnam Becker, and Lyot use stacks of uniaxial or biaxial plates having two or more different materials. Each plate has a particular thickness and relative azimuthal orientation. Each of these designs includes regions of high dispersion when used with ultraviolet light. This makes them difficult to describe and stabilize. Furthermore, these compensators require usage in a fixed arrangement with respect to the polarizing element, making them inappropriate for use within a rotating element ellipsometer. This is also the case for the compensating plate described by J. D. de Veer in U.S. Pat. No. 3,904,267, issued Sep. 9, 1975, and incorporated herein by reference in its entirety.
Biaxial crystals require a specific relation between the principle refractive indices to provide a wide field of view. This can be engineered into polymers, but these polymers do not reliably transmit ultraviolet light. The inventors are not aware of any suitable biaxial crystals.
Existing compensator designs have failed to enable a rotating compensator ellipsometry tool operating with a non-collimated, broadband illumination source that includes ultraviolet wavelengths. Thus, an improved compensator design is desired.